Field of the Invention
The invention relates to a method and an apparatus for producing semi-finished metallic products by remelting and/or remelt-alloying metallic materials, wherein solidification of the melt, i.e. re-solidification/quenching of the molten material at a high cooling rate (so-called quenching) is effected by means of a cooling device. Here, the processed materials are preferably disk, strip-, rod-, round rod-, wire-, bolt- or pipe-shaped initial semi-finished materials. The invention particularly relates to the production or processing of materials or semi-finished materials made of Nitinol (NiTi). In the following, the invention will therefore primarily be explained based on Nitinol, in which the invention preferably, but not exclusively can be applied.
Description of the Background Art
DE 1121281 A discloses a melting system for electric arc melting and electron melting of metals with reduced pressure, in which the metal is degassed. The molten metal drips into a cooled crucible and forms therein a molten pool which is kept liquid.
It is known that different phases can occur during solidification of a metal depending on the elements used and their ratio, see e.g. Gerhard Welsch, Rodney Boyer, E. W. Collings, “Materials Properties Handbook: Titanium Alloys,” ASM International 1993 and Gerd Lutjering, James C. Williams, “Titanium,” Springer 2003/2007 and Jan Frenzel, “Werkstoffkundliche Untersuchungen zur schmelzmetallurgischen Herstellung von Ni-reichen NiTi-Formgedächtnislegierungen,” Dissertation Ruhr University Bochum 2005, Shaker Verlag 2006. Furthermore, the presence of impurities such as oxygen and carbon may produce inclusions in the alloy formed during the melting process (see the literature cited). In metallurgy, inclusions are generally referred to as non-metallic materials in metals due to production or manufacturing.
The phases and inclusions often have an impact on the material properties such as strength, formability and fatigue behavior. Therefore, the controlled formation of phases and inclusions during the melting process can be used to achieve predictable and desired material properties. With respect to the influence of inclusions on the fatigue behavior of steels in general, see e.g. P. Grad, B. Reuscher, A. Brodyanski, M. Kopnarski and E. Kerscher, “Analysis of the Crack Initiation at Non-Metallic Inclusions in High-Strength Steels,” Practical Metallography 49 (2012) 468-469.
It is known to quickly quench metals or metal alloy systems from the melt by means of different methods in order to provide the material, when in the solid state, with properties that it does not have after slow casting and solidification processes, see for example the following references: Pol Duwez cited in U.S. Pat. No. 4,400,208 A; Pol Duwez, R. H. Willems and W. Klement Jr., “Continuous Series of Metastable Solid Solutions,” Journal of Applied Physics 31 (1960) 1136; US 2003/056863 A1 US 2009/139612 A1 US 2009/260723 A1 EP 0 024 506 A1 U.S. Pat. Nos. 4,537,239; 5,564,490; 5,842,511; 5,365,664; US 2004/0043246 A1. Thus, it is possible to transform metal alloys by rapid solidification into the amorphous state or to keep elements in solid solution (alloy supersaturation), which would already segregate upon slow solidification. Furthermore, a rapid cooling can cause alloys to solidify in a microcrystalline manner and prevent them from being precipitated as a course phase.
By rapid cooling from the melt, the formation of such equilibrium phases can be suppressed to a large extent. However, due to the limited thermal conductivity of many materials, there are geometry- or volume-related limited cooling rates such that the formation of extraneous phases can usually not to be suppressed to the desired extent.
A rapid solidification is achieved by transferring the heat content of a melt to a cooling medium being in contact with the melt by means of thermal conduction in a time as short as possible. Here, the quenching rate depends to a decisive extent on the size of the contact surface of the melt to the cooling medium in the ratio of the amount of the melt and the material of the cooling medium. The larger the contact surface in relation to the amount of melt, the higher is the quenching rate.
It is known from the prior art that quenching of molten metals can be done by means of melt spinning. See, for example, US 2013/0014860 A1, US 2012/0281510 A1, US 2007/0251665 A1 and WO 2000/47351 A1. Here, a thin stream of alloy melt is sprayed onto a rotating copper wheel serving as a cooling device. This produces thin, narrow strips. According to another known method, atomizing of a metal melt is performed into a cooling medium so that said melt is present as a fine powder.
The known methods have in common that they need to be followed by a further consolidation, i.e. mating of materials, for producing a compact material. It is possible to build up a body having a greater volume, for example by means of spray compacting. It is likewise possible to build massive bodies by selective powder melting (rapid prototyping). However, the known processes are usually subject to absorption of gas or undesirable remaining porosity.
In the case of Nitinol (NiTi), it is known to perform a heat treatment in order to shape the material or to influence the transition temperature or the elastic properties by a subsequent quenching process, i.e. to perform a rapid cooling. Therein quenching is not carried out from the melt, but with a solid body, i.e. this refers to solid phase processes and not to liquid phase processes. Examples thereof are WO 2013/119912 A1, US 2005/0082773 A1, US 2005/0096733 A1, US 2004/0059410 A1, U.S. Pat. No. 6,422,010 B1, U.S. Pat. No. 6,375,458 B1, JP 61106740 A, JP 61041752 A, JP 60169551 A, JP 60103166 A, JP 59150069 A and U.S. Pat. No. 3,953,253.
It is also known in Nitinol to produce very thin filaments or very thin strip material from a completely melted base material present in a crucible by means of rapid cooling. To that end, the liquid metal melt is applied to a cooled copper wheel or guided between two cooled rolls. In this case, a rapid cooling occurs, which is referred to as quenching, and the material solidifies. Methods of such type are described in JP 8337854 A, JP 5118272 A and JP 59104459 A, for example.
In Nitinol (NiTi), it has not yet been known to influence the metallurgical and mechanical properties of the material in form of a massive semi-finished product in a targeted manner, in particular the formation and size, the proportion and the distribution of extraneous phases in the massive semi-finished product by rapid cooling of the melt. Further, it has not yet been known to cool semi-finished Nitinol products out of the melt in a rapid active manner in the production.
According to the prior art, Nitinol is produced by various vacuum melting methods, namely VIM (vacuum induction melting) and VAR (vacuum arc remelting). In conventional EBR (electron beam remelting), for example known from the production of high-melting refractory metals such as tantalum and niobium, the melt solidifies in water-cooled copper crucibles, where the relatively large melt volume prevents a rapid solidification.
From the reference Mohammad H. Elahinia, Mahdi Hashemi, Majid Tabesh, Sarit B. Bhaduri, “Manufacturing and processing of NiTi Implants: A Review,” Progress in Materials Science 57 (2012) 911-946, it is known to use EBM (electron beam melting) for the production of Nitinol. In this case, a rod-shaped body of the base metals Ni and Ti is melted by an electron beam of high power with a large melting-off volume, wherein the body is at once melted over its entire cross-section. The molten material drips into a cooled copper mold and solidifies there due to its volume at a relatively slow cooling rate in order that the melt added does not encounter a fixed but a liquid material and can therefore form a homogeneous body. EBR with focused radiation has not yet been used for the manufacture of Nitinol.
When producing Nitinol, a multitude of undesirable extraneous phases and secondary phases in the form of binary, ternary and quaternary phases having different sizes and distribution forms in the solidification process in addition to the desired intermetallic primary phase, NiTi. They include, e.g. carbides such as TiC and the intermetallic phases Ti2Ni, Ti2NiOx, Ti4Ni2O and Ti4Ni3. These are commonly referred to as inclusions, because it is assumed that they have a very large influence on the material properties. Because of their constitution, the formation of these phases is almost inevitable and in particular favored by oxygen and carbon impurities which are introduced via the source materials (Ti and Ni) or originate from the process environment (crucible material or possibly surrounding atmosphere). In particular, oxygen and carbon impurities cause the phases Ti2NiOx and TiC. These are referred to as inclusions in the Standard ASTM F2063-5.
Recent studies confirm the assumption that the number, size and shape of such phases/inclusions in semi-finished products, e.g. pipes or wires, strongly influence the properties (e.g. corrosion resistance and fatigue behavior of Nitinol stents) of products made thereof; see, for example US 2010/0274077 A1. During subsequent shaping processes, around the inclusions that are more hard to deform often cavities (voids) form, which voids constitute additional weak points for corrosion, see, e.g. US 2012/0039740 A1.
Nitinol is an implant material. It is known that non-homogeneous structures and inclusions may reduce its corrosion resistance and durability; see, for example, the citations in U.S. Pat. No. 8,430,981 B1 by C. M. Wayman, “Smart Materials—Shape Memory Alloys,” MRS Bulletin 18 (1993) 49-56 and M. Nishida, C. M. Wayman, T. Honma, “Precipitation processes in nearequiatomic TiNi shape memory alloys,” Metallurgical Transactions A 17 (1986) 1505-1515 and H. Hosoda, S. Hanada, K. Inoue, T. Fukui, Y. Mishima, T. Suzuki, “Martensite transformation temperatures and mechanical properties of ternary NiTi alloys with offstoichiometric compositions,” Intermetallics 6 (1998) 291-301.
The influence of inclusions on the fatigue behavior of NiTi is described, for example, in Tak Ahiro Sawaguchi, Gregor Kausträter, Alejandro Yawny, Martin Wagner, Gunther Eggeler, “Crack initiation and propagation in 50.9 at. pct Ni—Ti pseudoelastic shape-memory wires in bending-rotation fatigue,” Metallurgical and Materials Transactions A 34 (2003) 2847-2860 and in M. Rahima, J. Frenzel, M. Frotscher, J. Pfetzing-Micklich, R. Steegmüller, M. Wohlschlögel, H. Mughrabi, G. Eggeler, “Impurity levels and fatigue lives of pseudoelastic NiTi shape memory alloys,” Acta Materialia 61 (2013) 3667-3686. Regarding the influence of inclusions on the corrosion resistance, see the reference Markus Wohlschlögel, Rainer Steegmüller and Andreas Schüßler, “Potentiodynamic polarization study on electropolished Nitinol vascular implants,” Journal of Biomedical Materials Research Part B: Applied Biomaterials 100B (2012) 2231-2238.
Especially with filigree implant structures (stents, heart valve frames) exposed to an additional corrosion fatigue by the body fluids, inclusions due to solidification have a negative effect on the fatigue and corrosion behavior. Therefore, the manufacturers of semi-finished Nitinol products make great efforts to produce Nitinol alloys with inclusions as low as possible, for example, by using high-purity raw materials, such as the so-called “iodide-reduced titanium crystal bar,” resulting in very high production costs.
By optimizing processes, the volume proportion of the inclusions could be significantly reduced in recent years, though interfering inclusions still occur, which can repeatedly be identified as fracture-triggering, especially in the fatigue behavior. The Nitinol produced by means of vacuum melting technology according to the prior art currently still contains undesired inclusions whose formation cannot be completely avoided, even when using very expensive, highly pure initial raw materials. Among experts there are still discussions regarding the effect of inclusions in Nitinol. Almost all technical alloys contain inclusions, and experts assume that Nitinol can not be melted without forming inclusions being omnipresent. It is believed that their size, distribution and nature can be influenced to a certain degree and that smaller, rounder and fewer inclusions may result in a improved fatigue behavior, but so far it could not be achieved in the prior art to produce Nitinol with no, almost no or very few, very small inclusions.
According to common standards, the volume proportion of inclusions and voids in medically used Nitinol may be 2.8% maximum and they may not be larger than 39 μm. Nowadays, technical further developments enable inclusion sizes between 10 μm and 20 μm. However, in particular the increasing miniaturization of medical implants (neurostents) and the increasing quality requirements (heart valve frames) require further efforts to improve the level achieved.